The present invention relates to a magneto which has a transistor ignition circuit for engines and which is adapted to prevent the engine from rotating reversely on starting.
Magnetos of this type comprise a rotor, such as a flywheel, provided with a magnet and a pair of poles, and a stationary assembly including a U-shaped iron core having a pair of legs and an ignition coil wound on one of the legs. The rotation of the rotor produces in the primary winding of the ignition coil a current, which is suddenly interrupted by a transistor, inducing a high voltage in the secondary winding to fire a spark plug. When the magneto is to be incorporated, for example, into a two-cycle engine, the air gap between the pair of core legs and the pair of poles must have a specified dimension. If the air gap is dimensionally inaccurate, there is the likelihood that the engine will start backward. Thus the magneto must be assembled with extremely high accuracy. More specifically stated, the ignition coil primary current has approximately the same wave form whether the rotor rotates positively or reversely, so that when the engine starts backward instantaneously, i.e. in the event of a kickback, a high voltage will be induced in the ignition coil at the very moment. If sparking occurs, the engine will continue reverse rotation.
Accordingly various devices have been proposed for preventing such reverse rotation. For example, a device is known which is adapted to electrically bring the magneto out of igniting operation upon detecting reverse rotation. Although the known device does not require high accuracy in assembling, the device necessitates an increased number of parts and is costly to make due to the use of the electric means.